A color CRT includes an evacuated glass envelope comprising a rectangular faceplate panel and a tubular neck connected by a funnel. An electron gun is located within the neck for generating and directing a plurality of electron beams through openings in a color selection electrode, such as a shadow mask or a focus mask, that is located in proximity to a luminescent screen provided on an interior surface of the faceplate panel. The color selection electrode is attached to a frame that is supported by springs which engage mounting studs that extend inwardly from the faceplate panel of the tube. A thin conductive layer, preferably of aluminum, overlies the screen and provides a mean to apply a uniform potential thereto. An internal magnetic shield, made of thin, cold-rolled steel, is fastened to the frame which is attached to the color selection electrode. The primary purpose of the magnetic shield is to reduce the influence of magnetic fields on electron beam trajectories as the luminescent screen of the tube is scanned by the electron beams. In particular, the angles of incidence of the electron beams at every point on the shadow mask must not change significantly from the design values. Otherwise, the beams will move away from the intended landing positions on the screen. The internal magnetic shield is designed to fit into the funnel and to be as close to the funnel wall as possible, without contacting a conductive coating on the interior surface of the funnel wall.
The CRT utilizes a flashable getter to provide an internal deposit, or film, of gas-adsorbing material essential for adequate life of the CRT. It is desirable that the getter deposit not be provided on an interior surface of the color selection electrode or on the luminescent screen. Getter deposits on the interior surface of the color selection electrode are undesirable because such deposits may overlie heat dissipative and/or X-ray suppressing coatings provided thereon, thus interfering with the operations of the coatings. Additionally, portions of a getter deposit directed onto the interior surface of the color selection electrode will pass through the apertures or openings therein and be deposited on portions of the aluminum layer overlying the luminescent screen. The getter material on the aluminum layer absorbs energy from the electron beams incident thereon, causing a decrease in screen brightness in the underlying areas of the screen. This results in an objectionable non-uniform appearance of the screen. To prevent the flashed getter material from being deposited onto the interior surface of the color selection electrode and the screen, it is known to locate the getter in proximity to an exterior surface of the internal magnetic shield, in order to restrict the deposit of the flashed, gas-adsorbing getter material to the exterior surface of the magnetic shield.
Internal magnetic shields may have any number of configurations, including substantially imperforate structures, or structures having open areas therein. The primary requirement, regardless of configuration, is that the internal magnetic shield provide magnetic shielding of the electron beams passing therewithin. Where the external surface of the internal magnetic shield also is used to support a deposit of the flashed getter material, care must be taken to ensure that none of the flashed getter material passes into the interior of the internal magnetic shield where it might be deposited directly onto the interior surface of the color selection electrode and onto the screen.
While it might seem desirable to utilize an internal magnetic shield that is imperforate, in order to restrict the getter deposit to the external surface thereof, it has been determined that when the shield is imperforate, a getter film deposited on the exterior surface is not readily accessible to the gas molecules within the tube. Consequently, an increase in localized gas pressure occurs, and positively charge ions are generated by the collision between the electron beams and the gas molecules within the tube. The positively charged ions are accelerated toward the cathodes of the electron gun, where they bombard and deplete the cathode coatings, resulting in the reduction of cathode emission. Thus, it is desired to have an internal magnetic shield structure that provides an exterior surface onto which the flashed getter material can be deposited, without allowing the getter material to pass into shield and onto the interior surface of the color selection electrode and the screen. The internal magnetic shield also must permit the gas molecules within the tube to readily reach the getter deposits on the exterior surface thereof, so that there is no deleterious effect on cathode emission.